What you're testing is if $newpasswd doesn't contain any characters from the range [A-Za-z0-9].

What you want instead, is testing if $newpasswd does contain any characters outside the character class [A-Za-z0-9], and then negate the test:

if ($newpasswd !~ /[^A-Za-z0-9]/) {
   # EXECUTE SOME THINGS;
}
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